1. Field of the Invention
The present invention relates to a method for determining a harmonic response of a device, and more 5 particularly, to a method for determining a more accurate harmonic response of a device over a dynamic range not limited by the stimulus source harmonic level.
2. Background
Harmonic measurements are of significant importance in many microwave, millimeter wave, and radio frequency (RF) applications including wireless communications. Excessive harmonic generation by components such as amplifiers or other nonlinear components in a communications device can lead to violations of spectrum rules set by the Federal Communications Commission (FCC), failed performance specifications, interference with other channels, or other problems. Harmonic measurements have been made by using a conventional spectrum analyzer, but this approach can be quite slow and the results are of only limited accuracy. Therefore, conventional methods of harmonic measurements using spectrum analyzers may be undesirable in a high throughput manufacturing environment in which both speed and accuracy of harmonic measurements are required.
To satisfy the requirements of speed and accuracy in harmonic measurements in a high throughput manufacturing environment, measurement techniques have been developed by using conventional vector network analyzers. However, a problem associated with conventional non-ratioed techniques for measuring the harmonic responses of a device by using typical vector network analyzers is that the internal signal sources of typical vector network analyzers are usually not very xe2x80x9cclean.xe2x80x9d The internal signal source of a typical vector network analyzer may generate a source harmonic in the range of xe2x88x9230 dB to xe2x88x9240 dB relative to the source fundamental frequency signal component. Although a source harmonic in the range of xe2x88x9230 dB to xe2x88x9240 dB relative to the source fundamental frequency component may not be regarded as a high harmonic level per se, the presence of such source harmonic can seriously affect the ability to accurately measure the harmonic response of a device. The presence of stimulus source harmonics can seriously limit the dynamic range of the measurements and the accuracy of the measurement results.
Therefore, there is a need for a method for measuring the harmonic response of a device with enhanced accuracy by using a typical vector network analyzer which may contain a source that has harmonics in addition to the source fundamental frequency component during the measurement of the device. Furthermore, there is a need for a method for measuring the harmonic response of a device to a fundamental frequency input with enhanced dynamic range that is not limited in measurement accuracy or dynamic range by the stimulus source harmonics.
In accordance with the present invention, a method is provided for determining the harmonic output of a DUT relative to the fundamental frequency output of the DUT. The method allows VNA calibration calculations to be made to eliminate source harmonics from DUT harmonic output measurements or at least to mitigate the effect of the source harmonics on the DUT harmonic output measurements.
Vector quantities used to determine the harmonic response of a DUT relative to an input fundamental frequency component are illustrated in FIG. 1. An output from the DUT is composed of two elements, the DUT""s harmonic response to a fundamental input from the source, and the DUT""s linear response to the harmonic input from the source. The vector sum of the DUT output responses, GHx, includes all composite harmonics from the DUT normally measured directly. The letter xe2x80x9cxxe2x80x9d in GHx represents a whole number, so if GHx is composed of second and third harmonics it will be a composite of GH2 and GH3. Harmonics from the source which are linearly passed by the DUT, GNx, are also readily measured with a VNA. In the method in accordance with the present invention, an output harmonic generated by the DUT, Hx, is calculated using vector subtraction according to the equation Hx=GHxxe2x88x92GNx. The output harmonic Hx will be free from source harmonic components and provide a more accurate measurement of parameters for the DUT.
When VNAs are used to measure harmonics in a conventional manner, a non-ratioed mode is used. But because of reliance of a VNA on ratioing to produce phase data, non-ratioed measurements can be noisy and inaccurate, and incapable of being used in a manner where the source harmonic contribution is characterized as a vector and mathematically removed. Accordingly, the method in accordance with the present invention provides for ratioing to obtain more accurate phase measurement results. To obtain the output harmonics Hx generated by the DUT relative to the source harmonic, the desired vector equation Hx=GHXxe2x88x92GNx can be used as described above. To obtain an output harmonic Hxxe2x80x2 relative to the source fundamental, the general Hx measurements are multiplied by a relative source harmonic level, that is, a ratio of a scalar harmonic measurement SH determined from a through line to a scalar fundamental measurement SF from the through line, or SH/SF. To obtain an output harmonic Hxxe2x80x3 relative to an output fundamental, the general Hx measurement is multiplied by the relative source harmonic level and then divided by the magnitude of a linear fundamental gain GN1 of the DUT as described in more detail to follow.